Excel Macro Help (autofill)

I'm in the process of writng an excel application to update a bunch of charts... I'm in the very beginning of the processs. where I'm taking searching column A for the first empty cell then inputting by month a new year (12 cells)... where i'm having trouble is the autofill to get the rest of the values filled in.

Private Sub cmdNewYear_Click()

Response = MsgBox("Are you wanting to input a new year ?", vbQuestion + vbYesNo, "Question")
'Message Box Asking if user wants to make a new year
If Response = 6 Then
'If Yes then
ThisWorkbook.Worksheets("Data").Activate
'Activate Worksheet named Data
Dim a As Integer
Dim b As Integer
a = 1
Do While Not IsEmpty(ActiveSheet.Cells(a, 1))
'checking to see if cells are empty then moving
to the next cell down col A
a = a + 1
Loop
ActiveSheet.Cells(a, 1).Select
'activate first empty cell
b = a + 12
a = a + 1
InputYear = InputBox("What year are you entering ?", "Enter Year", "1/1/20xx")
'input box for new year
ActiveCell.Value = InputYear
'input value from new year box into first empty cell
Cells(a, 1).AutoFill Cells(a, 1), Cells(b, 1)

One thing you might notice is that this does not fill in 12 months, but rather 12 days if the user sends a full date (i.e. "1/1/2003") through the input box. To remedy this, replace the autofill line with this for-loop:

For i = 1 To 12
Cells(a + i, 1).Value = DateSerial(Year(inputyear), i, 1)
Next i

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